1. Field of the Invention
The present invention relates to a nitride semiconductor laser device, and also to a nitride semiconductor laser device that is provided with a nitride semiconductor laser element having a high output.
2. Description of the Background Art
A high output semiconductor laser element can obtain a watt-level output of light. Such semiconductor laser element can be used in a processing field, a medical field, and the like using a laser beam.
Further, in recent years, development of a nitride semiconductor laser element that emits a purple-blue laser beam for pickup of an optical disk has been carried out, and it is coming to a stage of practical use. However, at present, among the semiconductor laser elements, a nitride semiconductor laser element that can obtain a watt-level output of light has not come to a stage of practical use yet. The wavelength of the purple-blue laser beam in the nitride semiconductor laser element is as short as about 400 nm. Therefore, the development of a nitride semiconductor laser element having a high light output is desired from a viewpoint of widening the application range of the processing field and the medical field using a laser beam.
Further, in recent years, an illumination device in which a semiconductor light emitting element and a phosphor are combined and used has been developed to use in place of a conventional illumination device such as an incandescent lamp or a fluorescent tube, and some of them are put to practical use. One example of the semiconductor light emitting element is a light emitting diode having a III-V group compound semiconductor as a light emitting layer. The light emitting diode can emit light of various colors such as red light, blue light, and white light.
An illumination device using a light emitting diode has advantages that it is small in size, inexpensive, low in power consumption, and long in life compared with a conventional illumination device such as an incandescent lamp or a fluorescent tube. However, an illumination device using the light emitting diode cannot obtain as large an output as the conventional illumination device. Then, at present, an illumination device using the light emitting diode is mainly used no more than as back light, an illumination, or an indicator of a display.
Here, in the case of using the above-described nitride semiconductor laser element having a watt-level output of light as a light source of the illumination device, the electricity-light conversion efficiency as a light source is extremely high compared with the case of using a light emitting diode as the light source of the illumination device, and it is expected that a drastically higher output can be achieved.
In the case of using a nitride semiconductor laser element as the light source, the light output that is obtained from the light source is about 0.3 to 1 W (for example, 0.5 W). Then, in order to realize stable operation at which the light output of the nitride semiconductor laser element is 1 W or more, it is necessary to electrically connect two or more nitride semiconductor laser elements on one sub-mount in plurality.
Further, in order to operate the nitride semiconductor laser element by stabilizing the light output of the nitride semiconductor laser element at 1 W or more as described above, it is necessary to apply a current of about 0.5 A per nitride semiconductor laser.
In addition, the voltage that is necessary to operate the nitride semiconductor laser element is 3.5 V to 6 V. On the other hand, the voltage that is necessary to operate a general semiconductor laser element other than the nitride semiconductor laser element is 1.8 V to 2.5 V. Therefore, the voltage that is necessary for the operation of the nitride semiconductor laser element is high compared with the voltage that is necessary for the operation of the general semiconductor laser element, and the power that is necessary for the operation of the nitride semiconductor laser element is 0.5 A×5 V or more.
At this time, a large part of the power that is not converted into the light output of the power that is input into the nitride semiconductor laser element is converted into heat. The heat raises the temperature of the nitride semiconductor laser element, and not only are oscillation characteristics such as the light output and the oscillation wavelength affected, but also the heat affects seriously the life of the nitride semiconductor laser element. Therefore, when the nitride semiconductor laser element is packaged in a package, or the like, it is considered to improve thermal radiation characteristics of the nitride semiconductor laser element. For example, an arrangement in which a p-n junction plane side of the nitride semiconductor laser element is bonded to the sub-mount, that is, a so-called junction down arrangement, exposing a back surface side of a substrate of the nitride semiconductor laser element, can be adopted. Further, a structure of a semiconductor laser device using a plurality of heat sinks has been developed (for example, Japanese Patent Laying-Open No. 2005-032937).
Further, the semiconductor laser device in which a plurality of the semiconductor laser elements are packaged in order to obtain a high light output is investigated in Non-Patent Document 1 (Electronics Letters, October 2005, Vol. 41, No. 21, pp. 1172-1173) for example. In Non-Patent Document 1, a plurality of semiconductor laser elements are arranged on the sub-mount in a state that they are not separated into individual elements.
FIG. 13 is a schematic drawing showing a configuration of the nitride semiconductor laser device that is conventionally investigated. The nitride semiconductor laser device shown in FIG. 13 is packaged with a so-called junction down arrangement in which the p-n junction plane side of a nitride semiconductor laser element 10001 is bonded to a sub-mount 10002. Here, sub-mount 10002 is bonded to a heat sink 10003 that is welded to a stem 10004.
FIG. 14 shows a thermal resistance circuit network in the nitride semiconductor laser device shown in FIG. 13. In the nitride semiconductor laser device shown in FIG. 13, generated heat 30001 in a light emitting region of the nitride semiconductor laser element passes through a thermal resistance 30002 from the light emitting region of the nitride semiconductor laser element to a sub-mount upper-side surface. Then, generated heat 30001 passes through a thermal resistance 30003 to a heat sink upper-side surface from thermal resistance 30002 to the sub-mount upper-side surface. Finally, generated heat 30001 is radiated from thermal resistance 30003 to the heat sink upper-side surface through a thermal resistance 30004 to a housing in which the nitride semiconductor laser device is stored.
Other than the thermal resistances shown in FIG. 14, a thermal resistance corresponding to a path in which the heat is radiated by natural cooling from the nitride semiconductor laser element and the surfaces of the sub-mount and the heat sink and thermal resistance corresponding to a thermal path between a plurality of the nitride semiconductor laser elements exist. However, because the thermal resistances that are not shown in FIG. 14 are normally negligibly small compared with the thermal resistances shown in FIG. 14, they are neglected.
FIG. 15 shows the current-light output (I-L) characteristic of the nitride semiconductor laser device shown in FIG. 13. The horizontal axis represents the current (I) applied to the nitride semiconductor laser device, and the vertical axis represents the light output (L) of the nitride semiconductor laser device. In the following, the relationship between the current and the light output is expressed as an I-L characteristic.
In FIG. 15, the nitride semiconductor laser device shows a linear I-L characteristic for a current value lower than a point 20001. However, it deviates from an extension of the linear I-L characteristic in the range from point 20001 to a point 20002. Then, the I-L characteristic from point 20001 to point 20002 shows that the increase rate of the light output to the applied current becomes small compared with the I-L characteristic up to point 20001.
The phenomenon shown in FIG. 15 has its origin in that a decrease of the light output is caused by increasing the temperature of the nitride semiconductor laser device due to the generated heat from the nitride semiconductor laser element. Furthermore, at a higher current value than at point 20002, deterioration occurs of the nitride semiconductor laser device originated from the generated heat from the nitride semiconductor laser element, and an increase in the light output of the nitride semiconductor laser device shows saturation. Then, even when a current higher than at point 20002 is applied, the light output of the nitride semiconductor laser device shows a tendency of decreasing due to the deterioration of the nitride semiconductor laser device originated from an increase in the generated heat from the nitride semiconductor laser element.